1. Field of the Invention
The invention relates to an apparatus that purifies exhaust gas discharged from an internal combustion engine. More particularly, the invention relates to an exhaust gas control apparatus that purifies exhaust gas discharged from a diesel engine using a particulate filter.
2. Description of the Related Art
Exhaust gas is discharged from internal combustion engines, such as gasoline engines and diesel engines, when they are operating. The exhaust gas contains substances that should not be discharged to the atmosphere. Particularly, exhaust gas discharged from diesel engines contains particulate matter (hereinafter, referred to as “PM”) that mainly contains carbon, soot, and soluble organic fractions (SOF). The PM, soot, and SOF cause air pollution.
Exhaust gas control apparatuses that remove PM contained in exhaust gas are known. Typically, such exhaust gas control apparatuses reduce the amount of PM discharged into the atmosphere from a diesel engine by passing the exhaust gas through a particulate filter provided in the exhaust passage and capturing the PM in the filter. Appropriate particulate filters used in such apparatuses may include, for example, diesel particulate filters (DPF) or diesel particulate-NOx reduction system (DPNR) catalysts.
The particulate filter becomes clogged as the amount of PM deposited on the particulate filter increases, which increases the pressure loss of exhaust gas passing through the particulate filter makes it more difficult for the exhaust gas to pass through the particulate filter. This increases exhaust back pressure in the engine, which reduces the output and fuel efficiency of the engine.
In order to solve these problems, the particulate filter is recovered by burning and removing the PM on the particulate filter in conventional exhaust gas control apparatuses. To burn and remove the PM on the particulate filter, for example, the amount of PM captured by the particulate filter (i.e., the amount of PM deposited on the particulate filter) must be determined. When the amount of PM captured by the particulate filter reaches a certain level, the temperature of the exhaust gas is increased. Also for example, Japanese Patent Application Publication No. JP 07-224636 A (hereinafter, referred to as “JP 07-224636 A”), Japanese Patent Application Publication No. JP 2004-019523 A (hereinafter, referred to as “JP 2004-019523 A”), and Japanese Patent Application Publication No. JP 2004-285882 A (hereinafter, referred to as “JP 2004-285882 A”) disclose a method of deciding the timing for recovering the particulate filter. In the method, a pressure difference sensor or the like detects the difference between the pressure on the upstream side and the pressure on the downstream sidle of the particulate filter, and the particulate filter is recovered when the detected pressure difference (the estimated amount of PM deposited on the particulate filter) exceeds a predetermined value.
In the exhaust gas control apparatus disclosed in each of JP 07-224636 A, JP 2004-019523 A, and JP 2004-285882 A, an upstream pipe and a downstream pipe are connected to a portion on the upstream and a portion on the downstream of the particulate filter, respectively. The pressure upstream and downstream of the particulate filter is introduced into the pressure difference sensor or the like through the upstream pipe and the downstream pipe.
In this configuration, the PM, soot, and SOF in exhaust gas enter the upstream pipe (pressure introduction pipe). The PM, soot, and SOF are deposited in the upstream pipe, which clogs the upstream pipe. Particularly, when soot is deposited, hydrocarbon (HC) that is unburned fuel in exhaust gas functions as a binder. Further, the exhaust gas is repeatedly pushed from a pressure inlet port of the upstream pipe toward the pressure difference sensor or the like, due to the pulsation of the exhaust gas pressure. As a result, a bent portion of the upstream pipe close to the pressure inlet port will very likely become clogged.
JP 07-224636 A discloses a method for preventing clogging of the pressure introduction pipe (upstream pipe). This method minimizes the amount of PM that enters the pressure introduction pipe due to the pulsation of the exhaust gas pressure by connecting the pressure introduction pipe to an exhaust pipe via pulsation-absorbing chamber. In this method, the pulsation-absorbing chamber, which generally has a complicated stricture, is required. This makes mass production difficult and also increases production cost. JP 07-224636 A also discloses a method for reducing the amount of PM that adheres to or is deposited in the pressure introduction pipe. In this method, the adhesion and deposition of the PM is reduced by inserting an end portion of the pressure introduction pipe into the exhaust pipe, which keeps the temperature of the end portion high using the heat of the exhaust gas. In this method, the structure for connecting the pressure introduction pipe to the exhaust pipe is complicated, which also makes mass production difficult, and increases production cost.